A photoelectric conversion element is used in an optical sensor or a photovoltaic device such as a solar cell. A photoelectric conversion element using dye-sensitized semiconductor fine particles is known in Patent Literature 1 or the like.
As the solar cell, a solar cell using a monocrystalline, polycrystalline, or amorphous silicon semiconductor is widely used, for example, in an electrical product such as a calculator and for residence. However, such solar cell using a silicon semiconductor is produced by using a high-precision process such as plasma CVD or a high-temperature crystal growth process, and hence the production requires a large energy and high production cost because the process requires an expensive apparatus which needs vacuum.
Therefore, there has been proposed, for example, a dye-sensitized solar cell using a material obtained by adsorbing a photosensitive dye such as a ruthenium metal complex to an oxide semiconductor such as titanium oxide as a solar cell which may be produced at a low cost. Specifically, for example, the dye-sensitized solar cell is produced so that an electrolyte solution fills a space between a negative electrode including, as a semiconductor layer, titanium oxide or the like having a dye made of, for example, a ruthenium complex adsorbed on the surface, on the transparent conductive layer side of a transparent insulating material such as a transparent glass plate or a transparent resin plate provided with a transparent conductive layer such as indium-doped tin oxide, and a transparent insulating material provided with a metal layer or conductive layer serving as a positive electrode and made of platinum, such as a transparent glass plate or a transparent resin plate. If the dye-sensitized solar cell is irradiated with light, in the negative electrode, electrons of the dye, which have absorbed the light, are excited and the excited electrons move to the semiconductor layer and are introduced to the transparent electrode, while in the positive electrode, the electrolyte is reduced by electrons from the conductive layer. The reduced electrolyte is oxidized by the transfer of the electrons to the dye, and the dye-sensitized solar cell is considered to generate an electric power by such cycle.
Currently, the dye-sensitized solar cell has low photovoltaic energy efficiency relative to an irradiated light energy compared with a silicon solar cell, and enhancement of the efficiency is an important problem in the production of an effective dye-sensitized solar cell. The efficiency of the dye-sensitized solar cell is considered to be affected by characteristics of elements that construct the solar cell and a combination of the elements, and a variety of attempts have been made. In particular, for a dye having a photosensitization effect, much attention has been focused on the development of a more efficient sensitizing dye. Ru dyes are now known as highly efficient dyes, but Ru is a transition metal and thus is expensive. Thus, much attention has been focused on the development of inexpensive and highly efficient dyes. Further, the dyes each have high photoelectric conversion efficiency in a visible light region but have low photoelectric conversion efficiency in a near-infrared region, and hence it is desired to develop a dye having an absorption band in the vicinity of the near-infrared region.
As organic dyes having such absorption band in the vicinity of the near-infrared region, some compounds are known in Patent Literature 1, Non Patent Literature 1, and Non Patent Literature 2. Dye-sensitized solar cells including a mixture of an organic dye having an absorption band in the vicinity of the near-infrared region and an organic dye having an absorption band in another region are disclosed in Patent Literature 2, Non Patent Literature 3, and Non Patent Literature 4. These literatures also disclose squarylium dyes.